Interpretive Summary: Several producers have observed injury in transgenic soybean from glyphosate under certain environmental conditions, and with certain salt formulations of glyphosate. A 2-yr field study was conducted by the Southern Weed Science Research Unit, Stoneville, MS to determine the effects of several glyphosate formulations on weed control and transgenic soybean (glyphosate-resistant) physiology and growth. Some glyphosate formulations caused temporary yellowing, but soybean recovered from the injury. Glyphosate accumulated in nodules of treated plants, and nodule development was temporarily inhibited. Soybean yields were generally higher with two glyphosate applications compared to one application regardless of glyphosate formulation. Results indicated that soybean injury and inhibition of nodule development with certain glyphosate formulations can occur, but soybean has the potential to recover from glyphosate stress.

Technical Abstract:
A field study was conducted during 2000 and 2001 at Stoneville, MS to determine the effects of isopropylamine (Ipa), trimethylsulfonium (Tms), diammonium (Dia), and aminomethanamide dihydrogen tetraoxosulfate (Adt) salt formulations of glyphosate on weed control, growth, chlorophyll content, nodulation, nitrogen content, and grain yield in glyphosate-resistant soybean, and to assess potential glyphosate accumulation in soybean nodules. Glyphosate was applied early postemergence (EPOST) at 0.75 kg ae ha-1 at one-trifoliate leaf or EPOST followed by late postemergence (LPOST) at 0.75 kg ae ha-1 at three- to four-trifoliate leaf to glyphosate-resistant soybean. Glyphosate-Tms and glyphosate-Adt injured soybean and visible injury ranged from 29 to 38% at 2 d after LPOST, however, soybean recovered from injury by 14 d after LPOST. Glyphosate salts had no effect on chlorophyll content of fifth- (9 d after LPOST) and ninth-trifoliate (23 d after LPOST) leaves, and soybean root and shoot dry weights at 14 d after LPOST. Glyphosate formulations had no effect on nodule number, but reduced nodule biomass at 14 d after LPOST compared to untreated control. Two applications of glyphosate-Ipa, glyphosate-Tms, glyphosate-Dia, and both applications of glyphosate-Adt reduced leghemoglobin content 8 to 10% compared to untreated control. Glyphosate levels in nodules from treated plants ranged from 39 to 147 ng g-1 (dry weight). The highest nodule glyphosate concentration was found with two applications of glyphosate-Ipa. Control of five predominant weed species at 14 d after LPOST was > 83% with one application and > 96% with two applications regardless of glyphosate salts. Soybean yields were generally higher with two applications compared to one application regardless of glyphosate formulation. Nitrogen content of fifth- and ninth-trifoliate leaves was similar among glyphosate formulations. Nitrogen content of soybean grain was slightly lower with two applications of Glyphosate-Ipa, glyphosate-Tms, and glyphosate-Dia compared to untreated control. These results indicate that soybean injury and inhibition of nodule development with certain glyphosate formulations can occur, but soybean has the potential to recover from glyphosate stress.